Intrinsic dielectric strength of dense pure hydrocarbons including polythene

Starting from the origin of ionisation by electrons colliding with gaseous hydrocarbons using the traditional Townsend approach, straight forward expressions are derived and illustrated graphically for the uniform field electric strength characteristic of dense methane and ethylene at their critical point, which exhibit the expected Paschen minimum though now at micrometer electrode spacings. Turning then to the more modern concept of molecule number density, further expressions for the intrinsic dielectric strength of all dense pure hydrocarbons, including polythene, are obtained which become independent of polymeric chain length for large molecules and these are also illustrated. Comparison between these two sets of characteristics show that whilst in gaseous/liquid methane and ethylene the electric field strengths are in the region of 100 MV/m. In dense/solid hydrocarbons like polythene these materials should be capable of withstanding fields of 1,000MV/m, which represent an upper limit. Some of the pitfalls preventing these high intrinsic dielectric field strengths from being reached, for instance the occurrence of high field electron emission from the cathode, are considered. With present day interest in short-fast-HV pulses, these high dielectric insulation strengths should become routinely achievable in modern electrical equipment.